A typical application where a fully automated door opening, holding, closing system is required is inside the paint booth of an automatic production plant. While painting a vehicle with a robotic manipulator, it is dangerous for a human to be inside its working envelope. During the painting cycle, doors must be opened and accurately held to a known position. This allows the robot to execute pre-taught paths to apply paint on the inside of the door and around trim locations consistently and without crashing into the vehicle door. Afterwards, the door must be closed to complete the paint application to the exterior and allow for the vehicle to index through the profile into and out of the booth.
One approach is illustrated in FIG. 1 wherein a two-position (either open or closed), non-servo, pneumatically controlled arm 10 pivots about an axis 11 with a gripper 12 which grabs special fixture or tooling 14 attached to a vehicle door 16 is shown. This is a conventional approach that the assignee of the present application uses in its paint booths.
The two position, non-servo controlled opening device, however, requires the attachment of fixtures to hold the vehicle door open. Fixtures are costly and require frequent maintenance. Also, the pivot axis of this device must be located directly under the door hinge. This area is typically one of the worst locations for overspray contamination. At this position the unit itself must be frequently cleaned and reliability decreases.
Another approach is illustrated in FIG. 2 wherein a retractable pin 18 is attached to a two-axis, electric, servo-controlled manipulator 20, which is inserted through the window slot of the car door 22. The manipulator 20 is generally programmable within the radius determined by the pivot lengths of its two arms combined. Determining the position of the door window slot prior to inserting the pin 18 on the manipulator 20 is achieved via a seek routine requiring a proximity switch to actuate as a hook approaches the door 22. This method of holding the door 22 does not require fixtures on the vehicle door 22 and does allow for the two-axis manipulator 20 to hold, open and close the door 22.
A similar approach is to use a retractable pin on a two-axis, electric, servo-controlled manipulator which uses the window slot for opening, holding and closing. The key difference is the original door window slot position, prior to inserting the pin, requires the tooling on the end of the manipulator to touch the inner panel of the door. Upon contact with the door the signal to drop the pin is executed.
One disadvantage of these two approaches is that the retractable pin with sensor is a costly method with some undesirable features. The pin contacts the door while holding it open. By virtue that it must contact the item being painted, the pin at some point is in direct line with the paint spray. This causes contamination of the pin and after a few paint cycles the pin requires cleaning.
Secondly, placing the pin in the window slot requires a sensing method. Typically, a door position, when it indexes into the booth, is not known within the tolerance zone required for placing a pin in a window slot. Therefore, the door position must be determined for each vehicle when it enters the booth prior to inserting the pin in the slot. Sensors add cost, reduce reliability and require additional cycle time to open a door.
A third approach is disclosed in the U.S. Pat. No. 4,946,336 to Larsson. Larsson utilizes a three-axis, servo controlled manipulator with a rare earth magnet. The magnet is mounted below the bottom edge of the door. The magnet draws the door over it, then uses a feedback control loop to get within a known distance and attitude (tilt) of the bottom edge of the door to generate enough force to open, hold and close the door.
The Larsson unit is the highest cost of the above-mentioned approaches for holding a door for painting. The Larsson unit requires a complex feedback loop because the gap between the bottom of the door and the magnet must be closely controlled to generate energy to handle the inertia and door detent forces experienced while opening and closing the door. Two servo axes are required just to achieve this feature. The third axis is dedicated to simulate the arc of the door. This requires the base of the unit to be placed underneath the door pivot hinge. This location exposes the unit to some of the worst overspray and contamination in the booth.
Another reason for requiring a controlling gap is to compensate for an inefficient use of the magnetic energy. To maximize the energy of the magnet holding force on the door, the magnet should follow the bottom edge of the door over the entire length of the magnet. There is no provision on the Larsson unit to allow the magnet to "find" the line of the door. Therefore, if the hinge of the door is not directly over the pivot of the base of the Larsson unit, the line of the magnet will not match that of the door at the open, held position. In a typical production environment, given the accuracy of the conveyor, body carrier, vehicle and body carrier positioners it is not unusual to find body positions varying between +/-25.0 mm.
The U.S. Pat. No. 4,498,414 to Kiba et al. and Kiryu et al. U.S. Pat. No. 4,546,724 both disclose a process robot having a tool extension thereon for opening and closing the door. However, there is no discussion of any mechanism for holding the door in its open position.
Other U.S. patents of a more general interest include U.S. Pat. No. 4,342,536 to Akeel et al; U.S. Pat. No. 4,743,159 to Inamori; U.S. Pat. No. 4,988,260 to Kiba et al and U.S. Pat. No. 5,014,644 to Yamamoto et al.